The invention relates to the enhancement of color by means of the optical interference effects which are produced by thin films. Interference phenomena in connection with thin films are well known. A summary of some of these phenomena is set forth in an article in the Scientific American entitled "Optical Interference Coatings", December 1970, pages 59-75. Although the article starts with a display of various colors in a colored illustration and includes references to certain color effects produced in nature by thin films such as oil slicks, soap films, oyster shells, and peacock feathers, the various scientific uses of optical interference coatings described in the article do not include the conscious production of visual color effects. A major use of optical coatings is the production of reflection or non-reflection across the visible spectrum. Thus anti-reflection coatings are used on lenses, and multiple reflective coatings are used in dielectric mirrors. Applications requiring enhancement at a particular wavelength have an analytical rather than a visual purpose and require the maximum reflectivity possible, such as the laser and the Fabry-Perot interferometer. Although the Plumbicon tube separates light into primary colors, these are not viewed, but produce signals for transmission to a receiver via megacycle carrier waves. Moreover, not only do these prior scientific uses of optical interference films have no visual purpose, but the way in which the films are used to achieve a particular effect is such that, once adjusted for this effect, the optical device in question can no longer be adjusted to control other parameters.
Various methods have also been used to alter the spectral transmission and other characteristics (absorption, color etc.) of materials such as glasses or plastics in order to make them useful as sunglasses, either as light absorbers to reduce and/or control the amount and nature of light reaching the eye, or for cosmetic reasons. These methods have included coloring the basic materials, adding a colored layer over the surface, adding a neutral filter to one or more surfaces, adding a polarizing material, etc.
However, the application of interference films to provide interference colors has not normally been used for such purposes. Such colors, although observed by many investigators, have not been used in general for cosmetic purposes because of difficulties in obtaining "predetermined" colors and because the colors lacked "depth", particularly on the transparent or partly absorbing substrates that are used for sunglasses and similar purposes. It is the purpose of this invention to show how such colors can be obtained having "depth" or color "density" under controlled conditions. In addition, this invention shows that such "high depth" colors can be obtained under conditions which allow the user to control the amount and nature of light transmitted to and through the substrate. This invention will also show how the latter control of the transmitted light can also be obtained while having "low depth" coloring. In fact, any practical degree and/or combination of color depth and transmitted light control can be obtained by proper use of the present invention.
In conventional optical techniques, interference films are commonly used to fabricate band-pass light filters and to "increase" (as distinct from the invention's effect, which is always to decrease the transmitted light, as in the case of sunglasses and other light reducing devices) the amount of transmitted light (for lenses, binoculars, etc.) through their use as so-called quarter-wavelength anti-reflection filters, the latter being a simple form of the former. As discussed below, since any film of "optical" thickness .lambda./4 (.lambda. being the wavelength of the radiation) is effective only around one value of .lambda. (or specific functions thereof), the application of such films having .lambda. values in the visible range causes the reflected and transmitted light components to be colored, even when the incident light is white, as is usually the case for sunglasses, windows etc.
By choosing film thicknesses properly, one can get a wide spectrum of reflected colors (the color of the transmitted light being the spectrum of the incident light, normally white, minus the reflected and absorbed components). This technique has not normally been used as a "coloring" mechanism primarily because of difficulties in controlling the color and very importantly because of the lack of intensity or color depth when used on transparent or partly absorbing substrates. In fact, such colors are normally observed only as a necessary adjunct to other factors such as the need for an anti-reflection filter on binoculars.
The lack of color depth (pastel shading in general) is acceptable for some purposes (e.g. lightly tinted sunglasses) but is not adequate for others. Another reason why interference techniques have not been put to widespread commercial use is the need to put such films on the outside of the lens (or window etc.) for best cosmetic effect or function. In practice, this means the films themselves must be quite hard or must be covered with another harder (normally transparent) film or layer to prevent scratching or other attack, thereby complicating the manufacturing process. The use of interference films has therefore been primarily restricted to optical instruments (binoculars, spectrometers, etc.) and techniques (band pass filters, etc.) where such factors are relatively unimportant because of the care which the optical components receive and/or the undesirability of or lack of need for coloration. In fact, in many scientific instruments which use interference effects for measurement purposes, monochromatic light must be used at some stage to provide the necessary operation. For most such purposes, conventional interference techniques are adequate.
However, in the case of plastic eyeglass lenses (both prescription and sunglass) there is a need for a coloring technique which can provide vivid cosmetic colors and also give protection to the soft plastic surface while providing the light reflection and/or absorption necessary to perform a worthwhile sunglass function. Similar applications exist in plastic windows, plastic decorator panels or building materials, etc. and also for other substrate materials (e.g. glass) in special applications (decorator panels or functional windows, etc.) Other applications will be obvious to those skilled in the art who become familiar with this invention. Some of such applications may simply require a color effect without the need to adjust other parameters such as light transmission. For example, such applications as plastic wall panels protected against scratching, costume jewelry, decorative dishes, bottles, and the like may incorporate the principles of the invention simply for a coloring effect.
In these applications, the interference coloring film must usually be extremely well bonded, to a degree not normally achieved with standard deposition techniques. Although any "appropriate" process capable of attaching the required materials in the "required" form to the substrate surface may be used in applying this invention, the invention itself has been demonstrated using ion beam sputtering and ion beam implantation sputtering techniques. The former is disclosed, for example, in U.S. Pat. No. 3,472,751. The latter is disclosed in my Disclosure Document No. 032867, filed Jun. 5, 1974, and can be used to deposit very tightly bonded, durable films on plastics and other difficult substrates, the film of deposited material commonly, but not necessarily, being harder than the substrate material.